1. Field of the Invention
This invention relates to an electrophotographic image forming apparatus, e.g., such as a photocopier, a laser printer, and the like.
2. Description of Related Art
Image forming apparatuses of this type have been known as electrophotographic photocopiers and laser printers, in which an electrostatic latent image is formed on an image carrier surface by exposing image light on the uniformly charged image carrier surface in accordance with an image scanned from an original or inputted as an image signal and in which a toner image is fixed and outputted after the electrostatic latent image is developed by a developer to form the toner image and the toner image is transferred onto a transfer material.
The residual toner remaining on the image carrier surface after the toner image is transferred is removed by a cleaning means and will be prepared for subsequent image forming processes. As a cleaning means, what is widely used is a plate shaped elastic member made in contact with the image carrier surface to wipe the residual toner off. The plate shaped elastic member is referred to as a cleaning blade, which is attached to orient in the opposite direction to a rotation direction of the image carrier, or namely in the counter direction, so as to wipe the residual toner off efficiently.
FIG. 16(a) is an illustration showing an essential structural portion of the image carrier and a cleaning means. In FIG. 16(a), a photosensitive drum 101 serving as an image carrier rotates in a direction of an arrow a, and the cleaning unit 102 is mounted in a counter direction in opposition to the rotation direction of the photosensitive drum 101. The cleaning unit 102 includes a cleaning blade 102a for wiping residual toner on a surface of photosensitive drum 101 by contacting with the drum surface, and a waste toner container 102b for collecting the residual toner as waste toner with the drum surface wiped by the cleaning blade 102a.
With the cleaning unit described above, the cleaning blade 102a may turn to extend in the rotation direction of the photosensitive drum 101 as shown in FIG. 16(b) due to contact friction between the cleaning blade 102a and the photosensitive drum 101. If the cleaning blade 102a is flipped over or turned over, the photosensitive drum 101 is unable to wipe off the residual toner on the surface of the photosensitive drum 101, so that extra toner may remain on the surface of the photosensitive drum 101 when an image is formed, thereby likely causing image defects.
Once the cleaning blade 102a is turned over, the blade's edge in contact with the drum surface likely sustains fine scratches even if the turnover of the cleaning blade 102a is corrected upon rotating the photosensitive drum 101 in the reverse direction to the arrow a in FIG. 16(b) or taking turnover of cleaning blade 102a is corrected upon photosensitive drum 101 out of the apparatus body, thereby causing some cleaning failures that the blade edge is partially unable to clean the drum surface adequately, and thereby impairing image quality by creating stripes extending in a conveyance direction on an image formed on a transfer material. Moreover, since excessive bending force applies to the cleaning blade 102a, permanent stresses may remain in the cleaning blade 102a itself, so that the cleaning blade 102a easily and frequently turns over, and so that partial cleaning failures may occur because the cleaning blade 102a contacts to the surface of the photosensitive drum 101 with uneven contact force that may vary depending on the location of the blade.
It has been known that such a turnover of the cleaning blade 102a frequently occurs when the photosensitive drum 101 is new and still not much used. A new photosensitive drum 101 has a smoother surface roughness, thereby enlarging the contact area to the cleaning blade 102a, and giving greater contact friction to the cleaning blade 102a. The cleaning blade 102a can turn more frequently at side ends of the blade rather than the center of the blade, and the entire area may turn over initially from a side end as the blade is being used while the blade end only has been turned over.
As a way to prevent the cleaning blade 102a from turning over, conceivable methods are to reduce contact friction in applying lubricant in advance to edge portions of the cleaning blade 102a to which the photosensitive drum 101 contacts; to reduce contact friction by abrading the surface of the photosensitive drum 101 with a fine sandpaper to make larger the surface roughness of the photosensitive drum 101 in advance. However, those methods require extra processes to apply a lubricant at the edge portion of the cleaning blade 102a or to abrade the surface of the photosensitive drum 101 with the sandpaper, and therefore, not only those methods invite increased costs but also those methods will not always bring adequate effects in terms of a way to prevent the cleaning blade from turning over.
It has been known that a resin as an original material for forming toner has a glass transition temperature around 40 to 80 Celsius degrees. If friction force generated between the surface of the photosensitive drum 101 and the cleaning blade 102a becomes too much, the edge shape of the cleaning blade 102a is transformed and thereby reduces capability of wiping residual toner. Furthermore, heat generated from friction between the cleaning blade 102a and the surface of the photosensitive drum 101 may increase as to raise the temperature at a boundary between the edge of the cleaning blade 102a and the surface of the photosensitive drum 101 up to around 40 to 80 Celsius degrees, thereby softening the toner located at the boundary. The softened toner is cooled upon reception of pressure onto the surface of the photosensitive drum 101, and a sticking phenomenon may occur in which the toner sticks on the surface of the photosensitive drum 101 in a semipermanent fashion. Because the surface of the photosensitive drum 101 is constituted of a material having a photosensitive property, the original photosensitive function of the photosensitive drum 101 may be impaired when the toner, an insulating material, is stuck on the surface of the photosensitive drum 101, and the photosensitive drum 101 may be therefore subject to a fatal malfunction.